1. Field of the Invention
The present invention relates to a light emitting device such as semiconductor laser, particularly a II-VI compound semiconductor laser, for short-wavelength emission, e.g., blue or ultraviolet laser emission.
2. Description of the Prior Art
There is a growing demand for semiconductor lasers which emit blue or ultraviolet radiation to meet requirements for high packing density and high resolution in the recording of data on and reproduction of data from optical disks and magneto-optic disks.
Semiconductor lasers for blue or ultraviolet emission are required to be made of a material having a large direct transition band gap Eg. Particularly, double-heterojunction semiconductor lasers are required to have a cladding layer made of a material having a greater band gap than the material of an active layer.
Semiconductor lasers have substrates on which various semiconductor layers are deposited by way of epitaxial growth. Such substrates should preferably be of a monocrystalline material of GaAS or GaP that is widely used in various compound semiconductor devices, and is highly crystalline, can be produced at a high rate, is available easily, and inexpensive.
Heretofore, II-VI compound semiconductor materials, especially IIb-VI compound Semiconductor materials or their mixed crystals, are promising as optical device materials because they are of a direct transition band structure.
IIa-VI compounds having a large indirect transition band gap Eg as fluorescent materials are also attracting attention. However, the IIa-VI compounds are unstable as they are hydrolyzed in air, and have unknown basic properties.
Therefore, it is considered advantageous to construct optical devices of IIb-VI compounds. However, it is difficult to select materials having different band gaps from a range of IIb-VI compounds and even their mixed crystals for active and cladding layers.
FIG. 1 of the accompanying drawings shows the relationship between the lattice constants and band gaps Eg of typical compound semiconductor materials. Since mixed crystals of IIb-VI compounds have large bowing parameters, difficulty has been experienced in combining materials which have large band gaps while achieving lattice matching relatively to each other.
Presently proposed semiconductor materials for emission in the blue wavelength range are ZnSe for an active layer and a superlattice of ZnSSe and ZnSe for a cladding layer, and ZnCdS for an active layer and ZnSSe for a cladding layer. These materials are however problematic in that the difference between the band gaps of the active and cladding layers is 100 meV or less, preventing the cladding layer from properly performing its function, i.e., optical and carrier confinement.
Japanese laid-open patent publication No. 1-169985 discloses a blue semiconductor laser comprising a substrate of GaAs, an active layer of ZnSe.sub.x S.sub.1-x, and a cladding layer of Zn.sub.x Mg.sub.-x Te. Japanese laid-open patent publication No. 63-233576 discloses a p-n-junction light-emitting device. The disclosed materials are based on MgTe, MgSe, and MgS crystals which have heretofore, been known as fluorescent materials. The MgSe, and MgS crystals are of an NaCl or hexagonal crystalline structure. It is impossible to grow these MgSe, and MgS crystals on a substrate of GaAs, ZnSe, or GaP which is of a zincblende structure.
For the above reasons, there have not been put to practical use any double-heterostructure semiconductor lasers having a band gap Eg.gtoreq.2.7 eV, and there have not been developed any semiconductor lasers having a substrate of GaAs, ZnSe, or GaP and capable of continuous excitation at room temperature.